Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells typically employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte or ion exchange membrane disposed between the two electrode layers. The membrane, in addition to being an ionically conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (i.e. fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or circulating coolant. The plates, which include the flow field channels, are typically known as flow field plates. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel stream that is supplied to the anode separator plate typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also used to control the temperature within a fuel cell assembly to prevent overheating. Conventional fuels cells employ a liquid, such as water, to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Thus, conventional fuel cells typically employ three fluid streams, namely fuel, oxidant, and coolant streams, which are fluidly isolated from one another. See, for example, U.S. Pat. No. 5,284,718 and U.S. Pat. No. 5,230,966, which are incorporated herein by reference in their entirety.
Fluid isolation is important for several reasons. One reason for fluidly isolating the fuel and oxidant streams from one another in a fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, the membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen. One reason for fluidly isolating the coolant fluid from the reactant fluids is to prevent dilution and contamination of the reactant streams. Water, which is typically used as a coolant, may cause flooding in the reactant fluid passages that prevents the reactants from reaching the electrochemically active membrane-electrode interface. It is also undesirable for the reactant streams to leak into the coolant stream because this reduces operating efficiency as the leaked reactants are not used to generate electrical power. One reason for preventing leakage of any of the fluids to the surrounding atmosphere is the general negative impact such leakage can have on fuel cell stack safety, performance and longevity.
Locating the source of fluid leaks has been found to be problematic. Once a fluid leak has been detected within a fuel cell stack (which is usually done by constant monitoring of the exhaust streams), locating the source of the leak is typically accomplished by disassembling the fuel cell stack into its constituent parts and testing each fuel cell individually. Such method is time consuming and, consequently, expensive. Furthermore, because the disassembling and individual fuel cell testing process can cause further damage/defects to the stack, such method can result in a worsening of the fluid integrity of a fuel cell stack.
Accordingly, there is a need for improved methods and apparatus for detecting and locating fluid leaks within a fuel cell stack. The present invention fulfils this and other needs and provides further related advantages.